New microwave method for measuring humidity in building materials

Humidity is the most controversial aspect of building health assessment. The waste of time, as well as the loss of the well-known drying and calcium carbide methods, limit their use in on-site testing. Conductivity and low frequency capacitance measurements are affected by salinity. Microwave detectors can overcome these difficulties. Therefore, it is believed that the microwave structure can be used for the measurement of water content. In addition, the miniature hygrometer with interchangeable probes is ready for different test depths. Finally, some application examples. Microwave moisture measurement, water content, surface probe, building materials
1 Introduction
Because commonly used methods of directly measuring humidity, such as drying, calcium carbide methods or Karl Fischer titration, require sampling from buildings, they are destructive and time-consuming. Therefore, these methods are not suitable for a closer inspection of the moisture distribution, or to locate the leakage through the humidity map. In addition, testing often requires the side of the material to be processed, just like a basement, so specifying the method of transportation or defining the size of the sample. Other established methods, such as conductivity and low frequency capacitance measurements, are affected by salinity due to long-term water diffusion or additives in building materials. Microwave frequencies in the megahertz range can often overcome this problem because the square root of the frequency will reduce the effects of salt. Therefore, the goal of this work is to select and optimize portable microwave structures suitable for humidity testing for building and construction materials.
The basic requirements for humidity detection are fast, easy to operate, one person can operate one test surface, light weight, fixed sample size, power stability and resistance to working conditions in the construction industry. In addition, the scanning or leakage location of building parts should store the measurement data in an orderly manner according to their position and transfer the data to the computer. Simple statistical methods should also be provided, such as averages. Microwaves, like all other methods, are affected by mechanical and chemical inhomogeneities in multilayer wall structures, trenches, different porosities, and rough surface qualities. Due to the internal structure or porosity and the effect of the coarse contact area, the interaction between the electromagnetic wave field and the wet material is large enough to balance the unevenness of a small range of materials. On the other hand, the degree of influence should be limited to a certain amount, area or depth that can be detected. Because these requirements are very different, a single microwave structure cannot meet this requirement. Therefore, the test setup for the replaceable sensor probe as shown in Figure 1 was selected. This setup includes a pocket-sized control connection probe and a sensor that displays measurement data and a variable humidity probe connected with a short cable. The probe contains a moisture-shaped planar microwave magnetic disk structure, including a metal part with high gripping force for high-frequency detection electronic components. Trigger the microwave sensor at a slightly lower position on the front end, keeping a certain distance from the sample. Figure 1: Pocket-sized measuring device and two interchangeable humidity probes
3. Selection of measurement method and microwave structure
The microwave moisture sensing method uses the change of the moisture content of the dielectric material to measure by analyzing the linear electromagnetic characteristics or the parameter changes within the range of the resonator and the antenna. For single-sided measurement of planar samples and balance of material composition and contact area non-uniformity within a certain range, only planar antenna and resonator structures are feasible. In the preferred microwave frequency band, the ISM microwave at 2.45 GHz, the influence range of the specially designed resonator is extended to some centimeter levels, and the influence of unevenness can be overcome in the millimeter range. The wave field of the antenna is affected by the length of electromagnetic attenuation within the range of wet material loss, usually at the decimeter level. Therefore, the principles of resonance and antenna structure are suitable for moderate measurements with building structures. 3.1 Linear resonant surface probe Linear resonator technology is particularly suitable as a humidity sensing device. Due to its inherent planar shape, it is resistant to deformation and simple to manufacture. The electric field between the front and bottom conductors is mainly concentrated inside the substrate. However, the interference field in the middle range of the millimeter distance is mainly due to the relative controlled line segment of the resonator. Therefore, expanding the range of the field is equivalent to allowing the distance to the center of the charge to reach the maximum distance of half the wavelength at microwave frequencies. (Independent substrate, 2.45 GHz, 3-5 cm). Due to the lower installation position of the triggered microwave structure in the probe, this part interacts with the sample through the air band, as shown in Figure 1. The balance of small unevenness can be achieved within the millimeter range. In addition, mode conversion caused by the loading of the resonator can be avoided. Through oscillating linear coupling or a larger range of ring coupling, a better average effect can be obtained. An effective average effective area of ​​2.45 GHz frequency ranges from 30 to 60 mm in diameter, which is small enough to address the moisture content of individual bricks. Figure 2 shows the front part of this linear resonant probe. Use a round chassis made of ordinary copper to close the probe. The chassis uses micro-linear technology and is divided into two symmetrical ring resonators. A resonant ring is a circuit that contains channels and coupling bands. The sensor probe is connected to the portable instrument with a short cable so that it can be used, for example, near the ground or on the ceiling, where it is not convenient for testing.
Figure 2 Front end of a linear resonant probe 3 Front end of a patch antenna probe The highly symmetrical resonator structure shown in Figure 2 can prevent the vibration of the sensor signal of the sample due to the reflection of radiated microwaves, which will be discussed in the antenna section below. Without the influence of radiation, the recording depth of the resonant probe can reach 1-3 cm, depending on the material and water content. Therefore, they are ideal for near-surface humidity measurement or inspection of layered structures, such as gray walls. Since the water content has reached equilibrium after diffusion, this can also provide an indirect distribution map for inaccessible, deeper, uneven building parts. 3.2 Patch antenna volume probe The use of radiating antenna structure can obtain a greater recording depth than non-radiating resonators. This can measure the internal water content of thicker walls or basements. Due to its flatness and simple manufacturing, the patch antenna in micro-linear technology, as shown in Figure 3, is very suitable for this purpose. In addition, due to the interaction of the near field, like the resonator, the antenna wants to emit electromagnetic waves in the sample. For the patch antenna located on the surface of the sample, there is a certain air band, and the point impedance depends appropriately on the dielectric constant of the sample. This is the basic role in use as a humidity sensor. The width of the air band can be used to adjust the interaction strength between the antenna and the sample material. However, the impedance of the feed point of the antenna also depends on the reflected energy of the back side of the received sample. Therefore, the sample should be thick enough to ensure sufficient attenuation of the reflected wave. This requires a sample thickness of 20-50 cm, a larger density value is lower, and a certain percentage of water content. Dry low-density substances must have a larger thickness, such as insulation materials. In addition, due to the scattering of the radiation beam, the sample area should be the same size. As shown in Figure 3, the basic structure of the patch antenna probe and the resonator probe are very similar. The only obvious difference is the flat front of the antenna, which improves the directional radiation pattern, indicating the smallest sample area. In addition, it ensures the distance between the fixed antenna and the sample.
4. Test and application
For both probes and various building materials, such as brick, porous concrete, sand stone or wood, the humidity can be investigated systematically and independently. Therefore, the network analyzer HP8752A is used to collect the sweep curve and moisture content of the complex reflection coefficient of all different substances. As a result, it was found that the change in resonance parameters was large enough to determine the moisture content of a selected fixed frequency material. The determination of the measured reflection coefficient can be related to the water content, the reference method for calibration by the drying method and the experimental data points. Figure 4 shows a resonant probe. The deviation measurement is due to the unevenness of the contact between the material and the surface. The numerical interpolation algorithm and calibration points are included in the microcontroller program code and can be run on the pocket hygrometer. Therefore, after selecting the substance of the menu-driven detection, the instrument directly gives the humidity ratio in the sample. In addition, the instrument can store test data in tables of various sizes. Because each measurement only takes a few seconds, parts of the building can be scanned in a short time. The average online speed is up to 16 measuring points, which improves the reliability of rough surfaces. Later data arrays can be transferred to the computer via a standard serial interface. Figure 5 shows the humidity distribution of a certain basement wall, using a patch antenna probe array measuring volume 20 cm wide. The three-point average of these data is almost at the same position. The leakage in the lower left corner and the higher humidity near the wall are clearly visible. Figure 4: Resonance probe function Figure 5: Humidity distribution in the basement wall
In the same detection area, the combined use of two probes can help locate the source of moisture. By comparing the near-surface water content and the water content of the leakage depth of the insulation area, it is possible to distinguish the condensation water caused by moisture.
5 Conclusion The structure of the specially designed planar microwave resonator and antenna is very suitable for portable hygrometer with compact probe. They complement each other well in terms of recording depth, average range and application field. The measurement accuracy of repeated detection or leak location is sufficient. The data array with short measurement time and ergonomic operation makes it possible to measure the humidity distribution map of the entire building.

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